1. Field
The present disclosure is generally related to minimizing cross-process direction non-uniformities of output images in printed documents caused by sensor misreadings using halftone patches.
2. Description of Related Art
An array sensor may be used in a printing device to scan images for output and thereby measure a wide variety of image defects that might occur in the xerographic or raster process. For example, as known in the art, the array sensor may scan a photoreceptor (e.g., in the form of a belt) to determine if toner is properly dispersed for output. The array sensor may be used to detect both non-uniformities in the cross-process direction and process direction (i.e., streaks and bands, respectively) for marking technologies. “Streaks” as used herein are defined as uniformity variations in the cross-process direction, at all spatial frequencies (i.e., “narrow” streaks as well as “wide” streaks including variations along the lateral side of the image printing system), and at all area coverage levels.
To correct for development induced streaks in a printing device, the array sensor monitors an amount of toner on the photoreceptor for each of the various halftones, or levels of coverage, for each color. For example, as is known in the art, one or more correction or test patches of halftone color may be provided on the photoreceptor. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor. Test patches are used to measure the deposition of toner on paper to measure and control the tone reproduction curve (TRC). Generally, it is known in the art that such measurements are used to correct the toner reproduction curve (TRC) for all pixels in the imaging raster.
Often, however, array sensors may incorrectly sense characteristics of the test patches which can result in the production of streaks and non-uniformities in the output image. For example, even though test patches contain many lines of raster output that are averaged, streaks and other non-uniformities may still be output. FIG. 2 shows a detailed view of a part of a sensed image 14 comprising a plurality of rows of halftone dots 16, 18, and 20 with non-uniformities due to butting error of the sensor chips. The non-uniformity in this case is caused by a sensor perceived overlap shown at 22 of halftone tons in rows 18 and 20. The perceived overlap 22 of rows of halftone dots 18 and 20 will appear as a darker line or streak to the sensor because such a concentration of dots appears to have a greater concentration of color. This perceived overlap will result in producing a lighter streak on a printed document, because TRC correction will attempt to lighten the coverage of halftone dots to compensate for the perceived reading. Alternatively, the sensor may perceive a lighter area or row of halftone dots, and thus induce a dark streak on a printed document after TRC correction.
For example, FIG. 3 illustrates an enlarged, detailed example of a halftone pattern 24 which may be used as a test patch. The halftone pattern 24 of FIG. 3 comprises a plurality of halftone dots 26 in rows or lines whose centers lie oriented (e.g., along an axis C) at an angle that is perpendicular to one or more sensors arranged in an array 30 along axis A (for reading in cross process direction B, as the photoreceptor moves in processing direction P). For example, for halftone patch 24 that is positioned at 90 degrees, the sensor may scan the lines/dots and sense the reflectance of test patch 24, and the sensor output may show a regular halftone pattern. This can be filtered, or averaged out in the cross-process direction, but only at the loss of some cross process spatial adjustment resolution for TRC correction. In addition, if a multi-chip array sensor is used, as shown in FIG. 3, there may be some spatial position error. For example, when the array sensor 30 comprises at least a first sensor (or chip) 32 and a second sensor (or chip) 34 linearly aligned along axis A to read in a cross process direction B, a slight gap 36 may be formed between the two sensors 32 and 34. Such a gap 36 may result in a disruption of the measured halftone frequency determined by each of the color patches. Non-uniformities such as overlap 22 in FIG. 2 may be caused by the gap 36 between at least first and second sensors 32 and 34, because halftone dots, such as those indicated in row 38, may not be sensed or may alternatively be perceived as being closer together. The application of TRC correction may then calculate a lighter area or a darker area for the printed document.
FIGS. 4-6 show in further detail how the sensor's readings may be affected. FIG. 4 shows an exemplary embodiment of an unfiltered profile graph of average output halftone frequencies for a typical normalized sensor response 42 for reflectance for a group of pixel positions or locations (numbers) 44. Each of the lines generally represents a different toner concentration or frequency that is read or sensed. Each of the spikes 46 may represent potential errors or streaks which may be formed in the output image. As shown in greater detail in the detailed example of an unfiltered profile of FIG. 5, a lower peak 45 of halftone frequency may be detected in certain pixel positions. The lower peak 45 may be a detailed view of one of the spikes 46 of FIG. 4, for example. Lower peak 45 may represent a potential butting error or non-uniformity read by a sensor array, such as the perceived overlap shown in FIG. 2. This indicates that there is less white on the photoreceptor (i.e., more toner) and that the sensor is missing the detection of the part without toner thereon (i.e., because of the gap 36). Even if such data is filtered, the butting error or non-uniformity can still be present in the frequency curve as shown by the dip or valley 47 in the filtered halftone frequency profile of FIG. 6. This indicates that the sensor/chip placement of the array sensor 30 has affected the reading of the amount of toner (or level of coverage) of the patch(es) in adjacent scan areas (e.g., due to the pattern, scan, or positioning error). Then, even more cross-processing filtering may be needed, with the results having less TRC correction resolution. Even with more filtering, errors or non-uniformities are still present in the output image.